The magnetic recording technique has been widely utilized in various fields, such as video equipment, audio equipment and computers, because of its excellent advantages (which are not seen in other recording systems), that is, it enables repeated use of media, it can easily use a signal in an electronic form, enabling the configuration of a system combined with peripherals, and it can modify a signal. In order to cope with the tendency toward miniaturization of equipment, the demand for improvement in the quality of recorded and reproduced signals, the demand for prolongation of recordable time and the demand for increase in the recording capacity, it has been desired to further improve the recording capacity of recording media.
To this end, improvements in magnetic materials have been made to improve the surface properties of the magnetic layer, the dispersibility of magnetic particles in the magnetic layer and the magnetic properties of the magnetic layer.
In audio and video applications, in order to provide a digital recording system having improved sound quality and picture quality as well as a video recording system adapted for high fidelity TV, magnetic recording media enabling the recording and reproduction of short wave signals have been required more than in the conventional systems.
Further, floppy discs having a magnetic layer on a flexible nonmagnetic support as external recording media for microcomputers and personal computers have been required to have a high capacity (as high as 10 M bytes or more) in order to cope with the recent spread of personal computers, the trend toward the improvement in application software, and the increase in the amount of data to be processed.
A recording system for high density code having a frequency component region 1.5 times wider than that of conventional codes, such as RLL signal, has been proposed for floppy discs. Thus, the shortest recordable wavelength of signals to be recorded on these floppy discs is coming close to 3.0 .mu.m or less, even 1.5 .mu.m or less.
In order to enhance the recording density of the system, the gap length of magnetic heads, too, has been, of course, reduced, nearing 0.5 .mu.m or less.
As a medium enabling a high density recording with the small shortest recordable wavelength, there has been proposed a so-called thin metal film type magnetic recording medium comprising a thin ferromagnetic metal film as a magnetic layer which has been recently put into practical use for 8-mm application.
However, the thin metal film type magnetic recording medium has many disadvantages in durability, running properties and corrosion resistance. Since such a magnetic recording medium has to cope with these problems, it cannot sufficiently provide its inherently excellent electromagnetic characteristics.
In the coating type magnetic recording medium comprising a magnetic layer mainly composed of ferromagnetic particles and a binder resin, it is necessary to reduce the size of ferromagnetic particles to be used or enhance the coercive force of the magnetic layer to provide a high density recording. As such an approach, there has been proposed the use of ferromagnetic metal powder as ferromagnetic particles in the magnetic layer particularly from the standpoint of coercive force in JP-A-58-122623 and JP-A-61-74137 (the term "JP-A", as used herein, means an "unexamined published Japanese patent application").
JP-B-62-49656 and JP-B-60-50323 (the term "JP-B", as used herein, means an "examined Japanese patent publication"), and U.S. Pat. Nos. 4,629,653, 4,666,770 and 4,543,198 disclose that as ferromagnetic particles there are used hexagonal system ferrites such as barium ferrite.
In a magnetic recording medium for computers (such as floppy discs), overwriting of signals having different recording frequencies is indispensable. The conventional media were enough if they allowed overwriting of two kinds of signals in a double-frequency relationship, i.e., 1f and 2f signals. However, the above mentioned RLL signal system requires not only the reduction of recordable wavelength but also overwriting of a plurality of signals having a frequency ratio of 3:8.
In the case where signals having a short recording wavelength and a great difference in recording frequency are thus used, overwriting of a signal with a shorter recording wavelength on a signal with a longer recording wavelength cannot be successfully performed if the magnetic properties of the magnetic layer are merely improved as disclosed in the above cited JP-A-58-122623 and JP-A-61-74137.
In other words, even when a signal with a shorter recording wavelength is overwritten on a previously recorded signal with a longer recording wavelength, the magnetic line of force doesn't reach deep in the magnetic layer and thus cannot erase the previously recorded signal with a longer wavelength.
In order to overcome this difficulty, it is most effective to reduce the thickness of the magnetic layer.
Also in video application, the reduction of the thickness of the magnetic layer is extremely favorable for the reduction of loss due to thickness or self-demagnetization loss and, hence, the enhancement of the electromagnetic characteristics.
If a magnetic layer is coated in a thin single layer, it is extremely unfavorable for the enhancement of the surface properties of the magnetic layer. In particular, when the thickness of the magnetic layer falls 1.0 .mu.m or less, it is difficult to obtain a magnetic layer having excellent surface properties on a nonmagnetic support in the form of a single layer.
Another problem associated with these magnetic recording media is electrification of the media. In other words, the occurrence of dropout due to the attachment of dust to the surface of the magnetic layer caused by the electrification of the media must be inhibited. In particular, in video tapes for use in digital video and magnetic recording discs which provide digital recording of computer data, the lack of recorded or reproduced signals due to dropout causes an increase in B.E.R. (bit error rate) giving a fatal problem.
The shorter the recording wavelength of signals to be recorded in these magnetic recording discs is, the greater is the effect of dropout on the magnetic layer. Thus, this electrification problem is a serious problem in the design of a magnetic recording disc having a large capacity and a high recording density.
An ordinary method of inhibiting the electrification of media is to incorporate carbon black in the magnetic layer. However, even if the thickness of the magnetic layer is reduced (taking into account the above mentioned problem caused by overwriting in recording of a short recording wavelength region), the amount of carbon black to be retained in the magnetic layer is limited. Further, in order to maintain the excellent magnetic properties, the incorporation of carbon black in the magnetic layer is preferably avoided as much as possible.
As another means of solving the electrification problem, there has been proposed a magnetic recording medium comprising a nonmagnetic layer containing carbon black or the like provided interposed between a magnetic layer and a nonmagnetic support. This magnetic recording medium is disclosed in, e.g., JP-A-55-55432, JP-A-50-104003, JP-A-62-214513, JP-A-62-214514, JP-A-62-231417, and JP-A-63-31027, and U.S. Pat. No. 3,440,091. The magnetic recording medium having such a layer construction is proposed also for the purpose of improving the surface properties or running durability of the magnetic layer.
However, the thickness of the magnetic recording media disclosed in the background art techniques is not sufficiently thin for short wavelength recording and high capacity media which are currently required.
When the thickness of the magnetic layer is reduced to improve the overwritability of magnetic recording discs, the amount of a lubricant which can be retained by the magnetic layer is reduced, deteriorating the running durability. In other words, after repeated sliding movement of the magnetic layer relative to the magnetic head, lubricant runs short, causing a rise in the friction coefficient thereof. As a result, the magnetic head will scratch the magnetic layer. If the magnetic layer contains too much lubricant, the physical properties thereof are deteriorated. Thus, the content of the lubricant in the magnetic layer is naturally limited.
If a ferromagnetic metallic powder or hexagonal system ferrite is used as ferromagnetic particles to be incorporated in the magnetic layer (as mentioned above) to increase the recording density and capacity of magnetic recording media, the magnetic layer tends to exhibit a deteriorated running durability.
Thus, a magnetic recording medium comprising a thin magnetic layer as an uppermost layer laminated on a nonmagnetic layer and a nonmagnetic support has been desired.
On the other hand, the thinner the magnetic layer is, the easier the magnetic layer can be peeled. The magnetic layer thus peeled off can be another cause of dropout. This problem, too, must be coped with. The peeling of the magnetic layer cannot be avoided even by providing a nonmagnetic layer between the magnetic layer and the nonmagnetic support. This problem occurs particularly in the case of the so-called successive multi-layer coating method which comprises coating a layer, drying the layer, and subsequently coating another layer.
This problem is considerably eliminated by using the wet-on-wet coating method as disclosed in JP-A-63-191315 (corresponding to U.S. Pat. No. 4,963,433) to coat a magnetic layer coating solution while a nonmagnetic layer (which has been coated on a nonmagnetic support) is wet.
However, in the wet simultaneous coating method, if the viscoelasticity of the underlayer coating solution is not close to that of the upper layer coating solution, the resulting magnetic layer is subject to linear coating marks or rough surface possibly because of irregular solution streams developed at the interface between the two layers, making it impossible to obtain a magnetic layer with excellent surface properties. In particular, the nonmagnetic layer coating solution and the magnetic layer coating solution differ from each other in the properties of particles dispersed therein and can hardly coincide with each other in viscoelasticity.
As mentioned above, in order to obtain a large capacity magnetic recording medium comprising a thin magnetic layer which provides a high density recording and exhibits excellent durability and running properties, the above mentioned various problems must be coped with. No means satisfying all these requirements have been proposed yet.